Tunnel freezers are well known in the art. The conventional tunnel freezer comprises:
(a) an elongated tunnel having a first end and a second end; PA1 (b) an item entrance port located at or near the first end for introducing items to be frozen into the tunnel; PA1 (c) an item exit port located at or near the second end for withdrawing the frozen items from the tunnel; PA1 (d) a conveyor belt for moving the items from the item entrance port, through the tunnel, and to the item exit port; PA1 (e) a refrigerant admission port located at or near either end for introducing a refrigerant into the tunnel; and PA1 (f) a refrigerant discharge port located at or near that end of the tunnel which is opposite from the refrigerant admission port for withdrawing the refrigerant from the tunnel.
See for example U.S. Pat. No. 4,800,728 by Klee.
Refrigeration systems for producing a refrigerated atmosphere in a tunnel freezer are also well known in the art. A state of the art system is the COLDBLAST.TM. fresh air freezing system taught in U.S. Pat. No. 5,267,449 by Kiczek et al. Kiczek teaches an open loop refrigeration system which uses air as the refrigerant. Through a process of compression, heat exchange and expansion, ambient air is cooled to approximately -250.degree. F. for delivery into the freezer at ambient pressure. A vacuum blower located downstream of the refrigerant discharge port provides for the withdrawal of the air refrigerant (now at approximately -100.degree. F.) from the tunnel at a subambient pressure. Subsequent to its withdrawal, the air refrigerant is processed in order to recover its remaining refrigeration by warming it against incoming air.
There is a concern, however, when a refrigeration system such as Kiczek's which provides for (1) delivery of the refrigerant at ambient pressure and (2) withdrawal of the refrigerant at a sub-ambient pressure is coupled with the conventional tunnel freezer. The concern is that as the pressure along the length of tunnel continually drops from the ambient pressure at the refrigerant admission port to the sub-ambient pressure at the refrigerant withdrawal port, a pressure gradient is created for outside air to leak into the tunnel. The pressure gradient gradually increases along the length of the tunnel until it reaches a maximum at the refrigerant withdrawal port. This pressure gradient for leaks is generally a concern only near the location of the refrigerant withdrawal port for two reasons. First, as noted above, this location is where the pressure gradient for leaks reaches a maximum. More importantly, however, this is also the location of either the item entrance port or the item exit port and such ports provide a ready access for outside air to leak into the tunnel. In addition to introducing heat into the freezer, such outside air also introduces moisture into the freezer which quickly turns to frost.
The tunnel freezer of the present invention addresses this concern by (1) locating refrigerant admission ports at both ends of the tunnel and (2) locating the refrigerant discharge port at or near the middle of the tunnel. Such a "dual flow" design confines the pressure gradient for leaks between the ends of the tunnel where it will not be a concern since the item entrance and exit ports (which ports, as noted above, provide a ready access for leaks), remain located at the ends of the tunnel.